It's not offensive, it's just unnecessary at the moment. Nor is it equal to the term 'black lives matter'.
The phrase isn't to make black people more important than white lives but to draw attention to the implicit and explicit injustices faced by black people all over the western world. Black people are unequal with regards to their treatment in many ways in comparison to white people.
If there was similar historical and present injustices faced by white people caused by a different dominant race then the term would hold equal weight. At the moment, it doesn't.
Again, this doesn't mean that white people or other races don't matter. That's just a caricature of the argument.